Direct Evidence for the Presence of Delayed Recombination in the Electroluminescence of ZnS

The primary and secondary peaks in an electroluminescent light wave in different pants of a ZnS crystal was studied to provide data for an explanation of the mechanism of the processes in one barrier. The electroluminescence was excited by square-wave voltage pulses of alternating polarity. The results show that the emission is concentrated in two peaks-one primary and one secondary- which however occur for voltage peaks possessing different polarity. A whole series of intermediate cases is also found. The voltage dependence of the local brightness is approximately equal to that of the total brightness. If there are several radiating stripes in the space between electrodes, their polarity as a rule alternates. A rectifying barrier may arise at a boundary between pure and doped material. The results confirm the idea of the periodic broadening of the barrier regions under the action of the external field. The processes occur in the following sequence: barrier broadening with formation of ionized activators in the barrier from voltage pulse in reverse direction, barrier narrowing to equilibrium width, exposure of ionized activators to current of returning electrons, recombination with rise of secondary emission peak, and combination of electrons with ionized activators from voltage pulse in forwardmore » direction which gives rise to primary peak. (J.S.R.)« less